Nonaqueous electrolyte secondary batteries for automotive applications such as power supplies for driving electric vehicles (EVs) and hybrid electric vehicles (HEVs and PHEVs) have a pressure-responsive current interruption mechanism as well as an explosion-proof safety valve. The pressure-responsive current interruption mechanism is activated by gas rapidly generated inside the battery in the event of an abnormal condition and interrupts the incoming current to prevent the battery from rupturing or catching fire.
One of the known techniques for increasing the capacity of nonaqueous electrolyte secondary batteries is to increase the charging voltage. Also known is a safety measure against the overcharging of nonaqueous electrolyte secondary batteries by the addition of overcharging inhibitors such as tert-amylbenzene, biphenyl (see PTL 1), cycloalkylbenzenes, and compounds having a quaternary carbon adjacent to a benzene ring (see PTL 2) to nonaqueous electrolyte solutions. Unfortunately, if the charging voltage is increased in order to improve the battery capacity, the overcharging inhibitor may decompose within the voltage range set as the normal operating range, depending on the type of overcharging inhibitor. This may result in poor battery characteristics and safety after charge-discharge cycling.
To solve this problem, the addition of lithium carbonate (Li2CO3) to positive electrode mixtures for nonaqueous electrolyte secondary batteries is also known, which improves overcharging resistance (see PTL 3). If lithium carbonate is added to a positive electrode mixture for a nonaqueous electrolyte secondary battery, carbon dioxide gas is generated from the positive electrode plate when a high voltage is applied to the battery, for example, upon overcharging. This allows the pressure-responsive current interruption mechanism to be reliably activated earlier than the explosion-proof safety valve.